The Parched Sea by TROY DENNING Forgotten Realms The Harpers #1 1991

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Determined to drive a trade route through Anauroch, the Zhentarim have sent an army to enslave the fierce nomads of the Great Desert. As tribe after tribe fall to the intruders, only a single woman, Ruha sees the true danger - but what Sheikh will heed the advice of an outcast witch?

Ruha finds help from an unexpected source. The Harpers, guardians of liberty throughout the Realms, have sent an agent to counter the Zhentarim. If she can help this stranger win the trust of the Sheikhs, perhaps he can overcome the tribes' ancestral rivalries and drive the invaders from the desert.